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This was too much to put as a comment, I shall remove this later or replace it with a solution.

@Cristian, could you try the following:

function my_meta_save($post_id) {
    // verify this is not an auto save routine. 
   if ( defined('DOING_AUTOSAVE') && DOING_AUTOSAVE ) return $post_id;

   //Verify nonce
   if (!wp_verify_nonce($_POST['my_meta_noncename'],__FILE__))
        wp_die('Nonce check failure');

    if ($_POST['post_type'] == 'page'){
        if (!current_user_can('edit_page', $post_id)) 
        wp_die('Permission check failure');
    } 

    $current_data = get_post_meta($post_id, '_my_meta', TRUE);  
    var_dump('<pre>'.$current_data.'</pre>'.);

    $new_data = $_POST['_my_meta'];
    var_dump('<pre>'.$new_data.'</pre>'.);

    //my_meta_clean($new_data); Ignore this while debugging
    if ($current_data) 
    {
         var_dump('Current data');
        if (is_null($new_data)) $ret =delete_post_meta($post_id,'_my_meta');
        else $ret = update_post_meta($post_id,'_my_meta',$new_data);
    }
    elseif (!is_null($new_data))
    {
        $ret = add_post_meta($post_id,'_my_meta',$new_data,TRUE);
    }else{
         wp_die('no data recieved');
    }
         wp_die('success:'.$ret);
    return $post_id;
}

Not tested